1. Field of the Invention
The invention relates to fixed cutter drill bits designed for abrasive applications, and more particularly to fixed cutter bits designed for high rate of penetration drilling in unconsolidated ultra abrasive formations.
2. Background Art
Different types of drill bits have been developed and found useful in different drilling environments. Bits typically used for drilling boreholes in the oil and gas industry include roller cone bits and fixed cutter. Cutting structures on bits vary depending on the type of bit and the type of formation being cut. Roller cone cutting structures typically include milled steel teeth, tungsten carbide inserts (“TCIs”), or diamond enhanced inserts (DEIs). Cutting structures for fixed cutter bits typically include polycrystalline diamond compacts (“PDCs”), diamond grit impregnated inserts (“grit hot-pressed inserts” (GHIs)), or natural diamond. The selection of a bit type and cutting structure for a given drilling application depends upon many factors including the formation type to be drilled, rig equipment capabilities, and the time and cost associated with drilling.
In drilling unconsolidated, ultra abrasive formations, bit life is limited due to excessive wear; therefore, bit cost has become a significant factor in the selection of bits for this environment. One example of an unconsolidated, ultra abrasive drilling application includes drilling of the pay zone of heavy oil reservoirs. Heavy oil reservoirs typically comprise unconsolidated to low compressive strength, yet highly abrasive sands that are permeated with thick, dense heavy oil. These dense, high viscosity liquid hydrocarbons are also sometimes referred to as bitumen.
Heavy oil production typically requires special oil recovery techniques, such as the injection of heat and/or pressure into the reservoir to reduce the viscosity of the oil and enhance its flow. One commonly used recovery technique is known as steam-assisted gravity drainage (SAGD), which involves drilling a pair of horizontal wells, typically one above the other, through the reservoir as shown in FIG. 12, wherein the upper well is used for steam injection into the reservoir and the lower well is used to produce the heavy oil. This is further described in Curtis, et al., “Heavy-Oil Reservoirs”, Oilfield Review, Autumn 2002, pp. 50.
Horizontal wellbores drilled through heavy oil reservoirs often extend 1000 meters or more through the reservoir. To maximize oil recovery in a larger reservoir, multiple directional wells may be drilled from a common wellbore to reduce the distance the oil has to travel through rock to reach a wellbore.
Drill bits used in unconsolidated, ultra abrasive applications are typically damaged beyond repair after a first run due to the extreme abrasion and erosion encountered during drilling. Milled tooth roller cone bits have been considered the most economically feasible bit for these applications because they cost significantly less than other bits and offer more aggressive cutting structures for higher ROP. Fixed cutter bits are generally not used in these applications because they cost 5 to 10 times more than a comparable roller cone bit and typically become damaged beyond repair after a first run, such that their higher cost can not be justified.
Although roller cone bits have been found to be most economically feasible for unconsolidated, ultra abrasive applications, the useful life of these bits is limited. As a result, several bits are typically required to complete a wellbore and the trips back to surface to replace the bits and the number of bits required to complete a well have a significant economic impact on a drilling program. However, up to now, milled tooth bits have still been found to be more economically feasible when compared to the significant cost of using a conventional fixed cutter PDC bit.
What is desired is a fixed cutter drill bit that offers increased useful life in high ROP, unconsolidated, ultra abrasive applications. In particular, such bits may be useful in reducing the number of trips required to complete wellbores in heavy oil drilling applications, or similar applications. Additionally, a drill bit capable of maintaining gage over an extended drilling operation in any highly abrasive environment is desired. Also desired is a more abrasive resistant drill bit that may be used to achieve higher rates of penetration (ROP) to provide a positive economic impact in a drilling program for a heavy oil drilling application.